1. Field of the Invention
The present invention relates to a mount for mounting a power unit on a vehicle body.
2. Description of the Prior Art
A known mount for mounting a power unit on a vehicle body is illustrated in FIG. 1 wherein an inner member 2 is mounted with an outer member 4 by means of two elastomeric arms 3 radially extending from the inner member 2 to the outer member 4. In mounting the power unit on the vehicle body, a plurality of such mounts are located between the power unit and the vehicle body. The outer member 4 is fixedly secured to the vehicle body and the inner member 2 to the power unit. The setting is made such that the two elastomeric arms lie in a horizontal plane upon application of a static load due to the weight of the power unit. The mount 1 comprises two elastomeric restraining blocks 5 for preventing an excessively large displacement of the inner member 2 relative to the outer member 4. In operation, a small vibration of the power unit in the vertical direction causes the inner member 2 to displace in the vertical direction, applying a shear stress to the elastomeric arms 3. The spring constant of the elastomeric arms 3 under shear stress is smaller than the spring constant of the elastomeric arms 3 under compression stress. An excessively large displacement in the vertical direction of the inner member 2 is prevented by abutting engagement of the inner member 2 with one of the two elastomeric restraining blocks 5.
The spring constant of the elastomeric arms 3 varies with variation in the direction of the vibration as shown by broken line curve A in FIG. 2 wherein Y-axis indicates the vertical direction and X-axis the horizontal direction. Referring to the broken line A in FIG. 2, the spring constant in the vertical direction is .alpha.(alpha) and the spring constant in a direction inclined from the vertical direction by an angle .theta.(theta) is a. Apparently, the spring constant in the vertical direction is minimum and the spring constant in the inclined direction is larger than the spring constant .alpha.(alpha) in the vertical direction.
It is necessary that the spring constant .alpha.(alpha) in the vertical direction be set considering the weight of the power unit and be large enough for supporting it. The spring constant in the inclined direction becomes large accordingly, failing to effectively prevent small vibrations in the inclined direction.
If, the mount 1 is arranged such that the two elastomeric arms 3 lie in a plane perpendicular to the inclined direction of vibration, the spring constant in the inclined direction reduces, thus effectively preventing the small vibration in this inclined direction. With this arrangement, however, one of the elastomeric arms 3 is subject to a tensile stress, while, the other to a compression stress. The elastomeric arm 3 which is subject to the tensile stress is liable to crack quickly as compared to the other elastomeric arm 3, leading to a short operating life.